Aircraft that fly today require a tubular framework to provide sufficient strength and rigidity to meet performance demands placed on such aircraft. In order to withstand the stress to which a modern aircraft is subjected, thin wall metal tubing must be bent to extreme angles without suffering damage which would weaken the tube at the point of such a bend. There are several bending methods commonly used in forming metal tubes into desired shapes. The most common of these bending methods are draw bending, compression bending, roll bending, and stretch bending. In draw bending, a workpiece is clamped against a bending form which rotates, drawing the workpiece through a pressure die, and if needed, over a mandrel. This method is very versatile and offers good control of metal flow for small radius and thin wall tube bending.
In compression bending a workpiece is wrapped around a stationary bending form by a moving wiper shoe. This method is desirable for bending rolled or extruded sections but metal flow is not as well controlled as in draw bending and is not practical for mandrel bends when there is more than one bend in the piece.
Roll bending is commonly used to bending round coils which are sometimes cut and the ends joined to form rings. This is impractical when there is more than one bend in a workpiece and is usually limited to heavy-walled tubing and large radii over six times the tube diameter. With this method, control of spring back and angles is difficult.
In stretch bending an entire workpiece is stretched longitudinally beyond its elastic limits and then wrapped around a form. This method reduces spring back, but is usually much slower than draw bending, compression bending and roll bending. This method is primarily used to form large irregular curves in sections that do not require mandrels.
Draw bending is commonly used to form tubes for the aircraft industry because the tubing used is generally thin-walled and the bends are generally small radius bends. A mandrel having a ball and socket linkage between ball shaped segments is commonly used in this process because balls can be added or removed easily; balls are also free to rotate and thereby distribute wear, and balls will flex in any plane thereby accommodating compound bends. While ball mandrels of this type have many good characteristics, such mandrels are expensive and are limited to specific tube diameters and wall thicknesses and because such mandrels are made of metal they are capable of damaging anti-corrosion coatings on the workpiece as a bend is being made.